Ultra large area (ULA) optical fibers are typically utilized in long distance transmission applications, for example, to reduce the number of required optical amplifiers at fixed distance between transmitter and receiver and/or to increase the number of optical amplifiers supported at fixed amplifier spacing. By launching signals with increased optical power into the fiber, less optical amplification is required, and therefore less noise is added to the signal. However, non-linear effects, including self-phase modulation, cross-phase modulation, cross-polarization modulation, four-wave-mixing, etc. can increase as a function of transmitted optical power density. The large effective area of the ULA optical fibers can help reduce the non-linear effects by providing a lower power density for a given amount of transmitted optical power. However, bending losses, which can attenuate the signal, can increase with increasing effective area. Therefore, optical fibers have been designed with special core and cladding index profiles, and with materials that can help reduce microbending and macrobending losses.
A variety of optical fibers are made with a depressed-index ring or trench region outside of the core of the fiber and within the cladding of the fiber. Such fibers can improve transmission properties including chromatic dispersion and bending losses. The trench fiber design was originally disclosed by Reed in U.S. Pat. No. 4,852,968, filed on Apr. 2, 1987. In U.S. Patent Application Publication No. 2007/0003198 by Gibson, a trench-assisted fiber is disclosed for reducing losses by controlling the power distribution in the central core and the annular region between the central core and the trench. U.S. Pat. No. 7,164,835 discloses a trench-assisted design for reducing the macrobending sensitivity of fibers. European Patent No. EP1978383A1 discloses a trench-assisted design for ULA fibers with improved macrobending sensitivity compared to ULA fibers without a trench. In U.S. Pat. No. 7,555,187, fibers with very large effective area are disclosed with acceptable macrobend loss. Yamamoto (Y. Yamamoto, et. al., “OSNR-Enhancing Pure-Silica-Core Fiber with Large Effective Area and Low Attenuation”, OFC 2010, paper OTuI2, March 2010) discloses a fiber with Aeff=134 μm2 and 1550 nm loss=0.169 dB/km. This fiber is designed with a depressed cladding index profile and the microbending sensitivity that is greater than 100 times that of standard single mode fibers. Bigot-Astruc (M. Bigot-Astruc, et. al., “125 um glass diameter single mode fiber with Aeff of 155 μm2,” OFC 2011, paper OTuJ2, March 2011) discloses a trench assisted fiber with 155 μm2 and microbending sensitivity that is about 10 times that standard single mode fibers but this fiber has 1550 nm attenuation that is 0.183 dB/km. Bickham (Bickham, “Ultimate Limits of Effective Area and Attenuation for High Data Rate Fibers,” OFC 2011, paper OWA5, March 2011) disclosed trench assisted fibers with Aeff about 139 μm2, but the microbending sensitivity is large and requires special low modulus coating materials to achieve low attenuation when the fiber is placed on the normal ship spools.
The prior art has provided a wealth of information on how to use the trench-assisted fiber design to make fibers with reduced losses or macrobending sensitivity, and to produce ULA fiber with an extremely large effective area. However, the prior art does not recognize or teach how to make trench-assisted ULA fiber that will provide optimum performance characteristics simultaneously across several key performance metrics.